This invention relates to a pipe-laying apparatus for laying a pipeline from a vessel, to a vessel including such an apparatus and to a method of conducting pipe-laying operations employing such an apparatus.
In shallow water “S”-laying is commonly used when laying pipeline, whilst in deeper water “J”-laying is more common. When laying pipeline by J-laying, various methods for holding the heavy weight of the pipe as it is lowered from the vessel along a tower are employed. In one known approach a combination of a travelling block and a pedestal is used whilst in another approach crawler track tensioners are used.
As a further safety measure, it is also known to provide additional safety clamps for gripping the pipe in the event that the usual gripping and lowering arrangement is inadequate or inoperative. Two safety clamps may be provided, one being movable a short distance along the tower and the other being at a fixed position so that by a suitable sequence of opening and closing the clamps and moving the movable clamp the pipeline can be raised or lowered, although at a much slower rate than during normal practice. Furthermore during laying of a pipeline from a vessel it may be necessary to lower an end of the pipeline into the water to abandon the pipeline or to recover the end from below water level. In either case a cable is connected to the pipeline by means of a special head which is welded to the pipeline, and commonly a winch of some kind is employed to raise or lower the pipeline. Where reference is made to a “cable”, it should be understood that the term “cable” is to be interpreted broadly and may include any form of flexible tension bearing member, including a fibre rope or a steel wire rope, for example, “full section” rope.
In either case of abandonment or recovery of the pipeline the cable from the winch has to accommodate very high tension loads. Often a double capstan winch and spooling device is used. Such an arrangement does not, however, avoid the need for a very heavy duty winch and results in long lengths of cable bearing very high tension loads, particularly when the cables run parallel to the deck, which is undesirable from a safety viewpoint. The problem is especially severe when laying a pipeline in deep water, especially when using a J-laying technique, because that results in even higher tension loads. It is then usually advantageous to have the cables extending along the tower down which pipe is laid. Providing such a winch arrangement in the tower presents significant feasibility and cost issues, because of the high loads involved, and because of the size of the winch and the need to provide adequate space and structural support in or near the tower to accommodate the winch. It is an object of the invention to provide a pipe-laying apparatus and a method of conducting pipe-laying operations that overcomes or mitigates the problems caused by using a winch for abandonment and recovery operations.